Donated blood is typically processed by withdrawing it through a needle and sending it through a disposable tubing set to a centrifuge in order to separate the blood into its various components. The centrifugal apparatus is fitted with a disposable plastic vessel through which the blood is circulated. The vessel is fitted into a centrifuge bowl that is driven by a motor. An exemplary vessel is a circumferential separation channel having several outlets positioned at different radial positions within the channel in order to remove blood components separated into stratified layers of differing density by the centrifuge. Red blood cells (RBC), being the most dense of the components, are packed within the channel at the most radially outward location, whereas the stratified layer of plasma is at the most radially inward location. A relatively thin, yellowish layer, called the buffy coat, contains white blood cells and platelets and is located between the red blood cell layer and the plasma layer. Within the buffy coat, the platelets are stratified toward the plasma, while the white blood cells are stratified toward the red blood cells.
U.S. Pat. No. 4,708,712, incorporated herein by reference, describes a two-stage separation channel for collecting platelets separately from white blood cells and also having an outlet for collecting source plasma. The red and white blood cells are returned to the donor, along with most of the plasma.
In a dual-needle procedure, frequently used with a centrifuge apparatus such as described above, whole blood is removed from the donor through a needle usually positioned in one of the donor's arms. The whole blood is then processed by the centrifugal apparatus and the white and red blood cells are returned to the donor through the tubing set and a needle usually positioned in the other arm. If only platelets are being harvested, more of the plasma is returned as well as the red and white blood cells.
For the separation channel described in U.S. Pat. No. 4,708,712, the dual-needle procedure is satisfactory for harvesting platelets in a manner that is relatively free from white blood cell contamination. It is, however, a somewhat difficult procedure for the donor since the donor must remain quiet for a significant period of time with needles in both arms.
In a single-needle process, blood is removed from the donor, processed to collect platelets, and the whole blood, minus the collected platelets and perhaps also minus collected source plasma, is returned to the donor through the same single needle. Platelets are stored in a platelet collection bag, and the plasma is separately collected and stored in a plasma collection bag. Processed blood from which the harvested components have been removed is stored in a separate blood storage return bag during the draw cycle. During the return cycle, a squeezing mechanism places pressure on the external sides of the blood storage bag in order to squeeze the blood from the bag for return to the donor through the single needle. A suitable blood storage bag and pressure mechanism is described in U.S. Pat. No. 4,991,743 which is incorporated herein by reference. In the single-needle process, as originally developed, the flow of blood through the centrifuge is halted for a specific period of time while blood is returned to the donor during the return cycle. As a result, blood flow through the centrifuge is intermittent and the interface between the plasma layer and the red blood cell layer shifts, causing significantly greater contamination of the collected platelets with white blood cells. In addition to greater contamination, the efficiency of collecting a significant percentage of the platelets from the donated blood is considerably less than the efficiency of the dual needle procedure. Thus, while the standard single needle intermittent flow procedure is more comfortable for the donor, it has less desirable results in the collection of platelets.
In order to improve the efficiency of the single-needle procedure and remove the contamination problem, a recirculating loop system has been developed in which the inlet pump is not stopped during the return cycle. Instead, the inlet pump, which pumps whole blood into the separation channel during the draw cycle, continues to operate during the return cycle to recirculate and reprocess blood already in the system. The recirculated blood, together with blood stored in the storage bag, are combined to provide both the recirculation flow to the inlet pump and the return flow to the donor.
In a prior art recirculation system, the draw cycle is operated for a period sufficient to withdraw and process a specific volume of whole blood from the donor. Once that volume has been obtained, the system is switched to a return cycle by opening a valve in the return line and putting pressure on the storage bag. Blood squeezed from the bag is returned in the reverse direction through the needle into the donor until the bag is essentially emptied, at which time the valve is closed and blood is once again drawn from the donor to the inlet pump. In this system, the instantaneous flow of blood to the donor in the return cycle is regulated in order to limit the amount of anticoagulant solution returned to the donor. To accomplish that end, the speed of the inlet pump is reduced during the return cycle and a programmable restriction valve is placed in the return line. In that manner the recirculation flow is related to the re-infusion flow in order to regulate the re-infusion flow to a desired instantaneous level. The time duration of the return cycle is lengthened to accommodate the need to regulate re-infusion flow and also to improve the efficiency of the process since additional platelets are harvested by reprocessing blood in the return cycle. A lengthy return cycle will reprocess more blood for a given recirculation flow.
The current invention seeks to maintain undisturbed the RBC interface of the buffy coat in the centrifuge throughout both the draw and return cycles while minimizing the length of time for the return cycle. It has been determined that it is not necessary to regulate the instantaneous flow of anticoagulant back to the donor as long as the average flow of anticoagulant returned over a complete single needle cycle is kept within the tolerance of the donor. As a consequence, a system has been developed in which the recirculation flow and the re-infusion flow are essentially independent during the return cycle. Instead, the recirculation flow is desirably established at as small a fraction as possible of the inlet flow during the draw cycle in order to minimize the duration of the return cycle and still maintain the stability of the interface. The re-infusion flow is regulated primarily by needle size; no restrictions are placed in the return flow line and the return line flow resistance is deliberately minimized.
By maintaining a stable interface at which the platelets are separated from the blood within the centrifuge, contamination of platelets by white blood cells is reduced to a level that is comparable to or less than that of the double-needle procedure. Platelet collection efficiency is improved by reducing inlet pump speed during the return cycle and thereby achieving an efficiency similar to or above the efficiency of the dual needle procedure.
While the prior art standard single-needle non-recirculation procedure is comfortable for the donor, the previous procedures can be used efficiently with only about 70% of the donor population. Various physiological considerations determine the maximum practical inlet flow rate--the size, the weight, the sex, and the hematocrit (red blood cell content) of the donor are important in determining the maximum practical flow rate for removal of blood from the donor by the inlet pump and for effective processing of blood by the centrifuge. The speed of the pump during the draw cycle is established according to those considerations. Since the total time for the procedure may be approximately an hour and 30 minutes, it is desirable to operate at the maximum practical flow rate for the specific donor. For the largest of donors, that may be about 90 milliliters per minute for the double-needle procedure. However, that flow rate corresponds to an average blood processing rate of only about 50 milliliters per minute for the standard non-recirculating single-needle procedure. The ratio, 5/9, is approximately the ratio of the duration of the draw cycle to the total cycle time for draw and return. As mentioned above, a prior art single-needle recirculating procedure desires long return cycles of perhaps even greater duration than 5/9 in order to control the return of anticoagulant. In this invention, it is desired to minimize the return cycle time in order to improve the average blood processing rate for a given inlet pump flow rate and thereby minimize the length of time for a completed donation.
With the return cycle duration minimized, a lower instantaneous flow rate during the draw cycle is possible for the same blood processing rate. Two additional benefits are achieved thereby: (1) when the instantaneous flow rate is lowered, the cell separation efficiency of the separation channel is increased thereby providing better separation and improved harvesting of the platelets; and (2) as the instantaneous flow rate is decreased, the percentage of the donor population which can be efficiently accommodated by the single-needle procedure is increased to about 98% of the donor population, thus providing the benefits of the single needle system to many more people.